Electron excited X-ray fluorescence device

ABSTRACT

A device for providing for electron excited x-ray fluorescence may include means for driving two contacting surfaces against each other in a low fluid pressure environment, such that high energy electrons strike a sample under test and provide for x-ray fluorescence of the sample. The sample under test may be in or on a sample holder, whose position with respect to the contacting surfaces is adjustable. For example, the sample holder may be positionable to be a different distances from the contacting surfaces.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S patent application Ser.No. 13/671,238, filed Nov. 7, 2012, the disclosure of which isincorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates generally to generation of high-energyelectrons, and more particularly to electron excited x-ray fluorescence(EXRF).

High-energy electrons may be used for medical or other materialexcitation applications such as etching or probing.

Other than through natural phenomena, high-energy electrons aregenerally generated by high voltage accelerators that require highvoltage power supplies. When electrons of a sufficient energy strike atarget or sample, they can dislodge an inner shell electron creating avacancy. When this vacancy is filled by the capture of outer shellelectrons, an x-ray with characteristic energy is emitted. This processmay be referred to as electron excited x-ray fluorescence (EXRF).Conventional x-ray fluorescence is carried out by the excitation ofinner shell electrons by x-ray irradiation. This process requires theuse of an x-ray source. X-ray generators are inherently inefficientbecause the conversion of high energy electricity to x-rays takes placethrough collisions, only a small fraction of which have sufficientenergy to result in x-ray radiation.

BRIEF SUMMARY OF THE INVENTION

Aspects of the invention provide for electron excited x-ray generation.

One aspect of the invention provides an electron excitation x-rayfluorescence (EXRF) device, comprising: a chamber maintainable at adesired fluid pressure; a vacuum pump coupled to the chamber, wherebythe vacuum pump may at least partially evacuate the chamber; a membranesufficiently transparent to x-rays within the chamber; a gapped rotordrivable to provide time varying contact with areas of a first surfaceof the membrane; a sample holder including a sample holding area forholding samples within the chamber during operation; and a sealable portin the chamber to provide access to the sample holding area.

Another aspect of the invention provides an electron excitation x-rayfluorescence (EXRF) device, comprising: a chamber; a vacuum pump coupledto the chamber to maintain the chamber at a desired fluid pressure; afirst surface and a second surface in at least partial contact with oneanother within the chamber, at least one of the first surface and thesecond surface being drivable so as to vary the portion of area of atleast one of the first surface or the second surface in contact with theother surface; a sample holder within the chamber, the sample holderpositioned in the chamber so as to have an at least partially clear lineof sight to an area of the first surface as areas of the first surfaceexit contact with the second surface; an electron source such as afilament of a Tungsten alloy, placed in the volume between the firstsurface and the sample holder; and an x-ray detector positioned so as tohave an at least partially clear line of sight to x-rays coming from thesample holder region.

These and other aspects of the invention are more fully comprehendedupon review of this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a cutaway view of a benchtop electron excited x-rayfluorescence device in accordance with aspects of the invention;

FIG. 2 illustrates aspects of operation of a device such as the deviceof FIG. 1;

FIG. 3 illustrates an example rotor useful in aspects of the invention;

FIG. 4 illustrates an example further rotor useful in aspects of theinvention;

FIG. 5 illustrates an example still further rotor useful in aspects ofthe invention;

FIG. 6 illustrates a sample tray adjustment mechanism in accordance withaspects of the invention;

FIG. 7 illustrates a further sample tray adjustment mechanism inaccordance with aspects of the invention;

FIG. 8 illustrates a cutaway view of a further benchtop electron excitedx-ray fluorescence device in accordance with aspects of the invention;

FIG. 9 illustrates an exploded view of portions of a device with avoltage potential mesh; and

FIG. 10 illustrates a sample tray with a voltage potential mesh.

DETAILED DESCRIPTION

FIG. 1 illustrates a cutaway view of an EXRF device in accordance withaspects of the invention. The EXRF device includes a rotor 111positioned to be in moveable contact with a membrane 113 during rotationof the rotor. The rotor may be mounted on a shaft 119, which is rotatedby a motor 117. In various embodiments the motor may be replaced withvarious other drive mechanisms, for example a magnetic drive assembly orthe like. The rotor and the membrane are in a chamber 121 bounded bywalls 123. The membrane is generally of an electrically insulatingmaterial, although in various embodiments the membrane may include metalor metallic components, and in various embodiments the membrane is orforms a dielectric material. In various embodiments the membrane is atleast partially transparent to x-rays, and in some embodiments themembrane is of a polyimide film such as Kapton or is of ethylenetetrafluoroethylene (ETFE). The chamber is maintained at a desired fluidpressure by a vacuum pump 125 coupled to the chamber by a port 127. Thedesired fluid pressure is generally less than 100 mTorr, or in variousembodiments 50 mTorr, 1 mTorr, or 0.001 mTorr. Also within the chamber,rearward of the rotor, with the membrane considered to be forward of therotor, is a sample tray 115. Access to the sample tray may beconveniently provided by way of a sealable door 133 in a wall of thechamber. In some embodiments position of the sample tray is adjustablerelative to position of the membrane.

Moving contact of portions of the rotor, which may be of metallicportions of the rotor, and the membrane results in portions of the rotorbeing in time-varying contact with varying areas of the surface of themembrane. The moving contact may be, in various embodiments, frictionalcontact, sliding contact, or rolling contact. In such circumstances, thevarying contact results in relative electrical charging between thecontacting materials, where the materials are chosen so that themembrane becomes negatively charged. This charge imbalance sets anelectric field with a potential proportional to the magnitude of thesurface charge density. Electrons, and/or in some embodiments ions,within this field will accelerate away from the surface with the samesign of charge, towards surfaces of opposite or lesser charge. Theenergy attainable by the charged particles is generally controlled bytheir initial position and the distance between the charged surfaces.Some of the accelerated electrons may strike portions of the rotor andsome of the electrons may also strike a sample in the sample tray. Asufficient charge density will accelerate electrons such that theircollisions will result in the emission of x-rays.

In some (but not all) embodiments an electron source 120 is provided inthe vicinity of the membrane. As illustrated in the embodiment of FIG.1, the electron source is an exposed metallic filament, for example of atungsten alloy, in some embodiments. As illustrated in the embodiment ofFIG. 1, the filament is coupled by a wire to control electronics 131,which provides current to the filament, and for example controls theflow of electrons. The control electronics may provide, for example a 2Amp current to the filament, for example using a 3 Volt voltage supply.The position of the filament can be chosen to control the location ofthe source of electrons. In some embodiments the filament could be at afixed position. In other embodiments the filament could be mounted on acontrollable stage, for example to vary relative position of thefilament with respect to the contact surfaces or the sample holder.

The electrons striking the sample cause x-ray fluorescence of thesample, with the sample generated x-rays having characteristicsdependent on elemental composition of the sample. The x-rays pass to adetector 129, which in the embodiment of FIG. 1 is also within thechamber. The detector is coupled to control electronics 131. The controlelectronics may process signals from the detector, command operation ofthe vacuum pump and the motor, and provide signals to external memoryand/or displays.

In some embodiments the membrane extends from a spool (not shown), withmaterial of the membrane also wrapped on the spool. As membrane materialbecome worn, or as desired, additional material may be unwound from thespool to provide a fresh contact surface for blades of the rotor. Insome embodiments the spool is within the chamber 121.

In some embodiments the x-ray detector is positioned in other positions,for example to the side of the membrane and sample holder, as viewed inFIG. 1. In such embodiments the vacuum pump and control electronics maybe appropriately repositioned to make room for the x-ray detector, andthe shape of the chamber 121 may be adjusted for such a configuration orthe x-ray detector may have an exposure to the chamber by way of awindow, for example a beryllium window. In some embodiments two x-raydetectors may be used, with for example one positioned as shown in FIG.1 and another positioned to the side of the membrane and sample holder.

In some embodiments the sample holder may be transparent orsubstantially transparent to x-rays. In some embodiments the sampleholder may be opaque to x-rays. In some embodiments the sample holdermay contain certain predetermined elements. The use of predeterminedelements in the sample holder may be beneficial in that operation of thedevice, without samples, would be expected to provide x-ray emissionscharacteristic of the predetermined elements, allowing for ease ofcalibration of the device. For example, prior to testing of samples, thedevice may be operated with an empty sample holder to obtain a baselineresponse of the device. In various embodiments the baseline response maythereafter be subtracted from operational responses obtained duringtesting of materials.

FIG. 2 illustrates conceptual operation of a device such as the deviceof FIG. 1. In operation, a motor 211 rotates a drive shaft 213. A rotor215 is mounted to the drive shaft, with blades of the rotor extendingaway from an axis of rotation of the drive shaft. The blades of therotor are in contact with a membrane 217, with the blades sweepingacross areas of the membrane. As the blades clear any particular area ofthe membrane, the negative surface charge in the membrane is exposed.Free or weakly bound electrons in the vicinity of the membrane willaccelerate away from the membrane 219. An electron source such as aTungsten alloy, can be used to generate free electrons in the vicinityof the membrane, for example as discussed with respect to FIG. 1. Atleast some of the electrons travel away from the membrane, past therotor, and strike a sample in a sample tray 221. This results inelectron excited x-ray fluorescence of the sample, with x-rays 223impinging on a detector 225.

FIGS. 3-5 illustrate example rotors mounted on drive shafts inaccordance with aspects of the invention. In FIG. 7, a pair of wings 313a,b extend away from each other about a distal end of a drive shaft 311.In FIG. 4, a vane 413 extends from a distal end of a drive shaft 411,with an optional additional vane 415 also possible. In FIG. 5 a pair ofrollers 513 a,b are rollably coupled to a drive shaft 511 by a wire 515or other mount. In operation the rollers roll over areas of a membraneor other contact material.

FIG. 6 illustrates a portion of a device such as the device of FIG. Iwith an adjustment mechanism for adjusting position of a sample tray 617with respect to a membrane 613. Generally the adjustment mechanism maybe a height adjustment mechanism. In FIG. 6, blades 611 of a rotorcoupled to a drive shaft 615 brush against the membrane during rotationof the drive shaft. Electrons generated by this operation strike asample on the sample tray, resulting in electron excited x-rayfluorescence of the sample.

Position of the sample tray relative to the membrane is provided byadjustment rods 619 a-b, which may be raised or lowered with respect toa base of a chamber including the sample tray and the membrane. In someembodiments the adjustment rods are telescopic assemblies, which may beextended or retracted to provide for different distances between thesample tray and the membrane.

FIG. 7 illustrates a further example of a device providing foradjustable position of a sample tray with respect to a membrane. Asshown in FIG. 7, opposing walls of the chamber include a plurality ofpairs of slots, for example a first pair of slots 719 a,b and a usedpair of slots 721 a,b. A sample tray 717 is placed in the used pair ofslots, although the sample tray may instead be placed in any of theother pairs of slots. As the pairs of slots define positions at varyingdistances from the membrane, the slots provide for adjustablepositioning of the sample tray with respect to the membrane.

FIG. 8 illustrates a cutaway view of a further EXRF device in accordancewith aspects of the invention. The embodiment of FIG. 8 is similar tothe embodiment of FIG. 1. As with the embodiment of FIG. 1, The EXRFdevice includes a rotor 111 positioned to be in moveable contact with amembrane 113 during rotation of the rotor. The rotor may be mounted on ashaft 119, which is rotated by a motor 117. The rotor and the membraneare in a chamber 121 bounded by walls 123. The chamber is maintained ata desired fluid pressure by a vacuum pump 125 coupled to the chamber bya port 127. The desired fluid pressure is generally less than 0.001Torr. Also within the chamber, rearward of the rotor, with the membraneconsidered to be forward of the rotor, is a sample tray 115. Access tothe sample tray may be conveniently provided by way of a sealable door133 in a wall of the chamber. In some embodiments position of the sampletray is adjustable relative to position of the membrane.

During operation, x-ray emissions from samples in the sample tray passthrough the bezel 825 to impinge on the detector 827. The detectorprovides signals to control electronics 831.

FIG. 9 illustrates an exploded view of portions of an EXRF device inaccordance with aspects of the invention. In FIG. 9, a bladed rotor 911is mounted to a motor shaft 913 driveable by a motor 915. A sample tray917, having a slot for insertion about the motor shaft, is positionablebetween the motor and the rotor. A wire mesh 919 is positionable over atop of the sample tray, for example as may be seen in FIG. 10. The wiremesh may, in some embodiments and in some applications, serve torestrain or retain samples in the sample tray.

In some embodiments, the wire mesh is coupled to a voltage source, withthe wire mesh maintained at some desired potential. The desiredpotential may be sufficiently positive to attract free electrons, forexample generated in the embodiments of FIG. 1 or 8, towards the sampletray. In some embodiments the desired potential is sufficiently positiveto attract electrons towards the sample tray, but not so positive thatthe wire mesh is so attractive to electrons that the sample receivesfewer electrons than the sample would receive if the wire mesh were notpresent, or was not maintained at any particular potential.

Although the invention has been discussed with respect to variousembodiments, it should be recognized that the invention comprises thenovel and non-obvious claims supported by this disclosure.

What is claimed is:
 1. An electron excitation x-ray fluorescence (EXRF) device, comprising: a chamber maintainable at a desired fluid pressure; a vacuum pump coupled to the chamber, whereby the vacuum pump may at least partially evacuate the chamber; a membrane of a dielectric material within the chamber; a gapped rotor drivable to provide time varying contact with areas of a first surface of the membrane; a sample holder including a sample holding area for holding samples within the chamber during operation; and a sealable port in the chamber to provide access to the sample holding area.
 2. The device of claim 1, wherein the sealable port comprises a door in a wall of the chamber.
 3. The device of claim 1, wherein the sealable port is a slot in a wall of the chamber.
 4. The device of claim 3, wherein position of the sample holder is adjustable with respect to position of the membrane.
 5. The device of claim 3, further comprising means for adjusting position of the sample holding area with respect to the membrane.
 6. The device of claim 1, wherein a distance between the membrane and the sample holding area is adjustable.
 7. The device of claim 1, wherein the sample holder is substantially opaque to x-rays.
 8. The device of claim 1, wherein the sample holder is substantially transparent to x-rays.
 9. The device of claim 1, further comprising a mesh over a sample tray.
 10. The device of claim 9, wherein the mesh is maintained at a predefined electrical potential. 